Oil return algorithm for capacity modulated compressor

ABSTRACT

A control for a compressor motor is programmed to operate the motor at at least one low capacity and at least one higher capacity. The control is operable to estimate an amount of oil which will have migrated from said compressor shell, at least when the compressor is operating at the lower capacity rate. The motor control moves the compressor motor from the lower capacity to at least one higher capacity once the estimated quantity of oil exceeds a predetermined limit. The capacity may be related to space.

BACKGROUND OF THE INVENTION

This application relates to an algorithm for ensuring adequate oil return in a refrigerant system including a capacity modulated compressor.

Compressors are utilized as an integral part of a refrigerant system. In general, a compressor compresses a refrigerant and passes it downstream to a condenser. Refrigerant from the condenser passes through an expansion device, and then through an evaporator. From the evaporator, the refrigerant returns to the compressor.

Lubricant is included in the refrigerant system, typically an Air Conditioner, Heat Pump or refrigeration application, and is particularly important to lubricate moving parts in the compressor. However, the lubricant becomes entrained in the refrigerant, and can flow with the refrigerant throughout the refrigerant system. As such, there may sometimes be an inadequate supply of lubricant returned to the compressor. Lubricant can sit in other areas of the refrigerant system, and in particular in the condenser and evaporator.

Recently, system energy efficiency improvements have resulted in the compressor and refrigerant systems being of a variable capacity. Typical applications of the compressor's ability to vary capacity include variable speed, multi-stepped modulation, PWM of compression element engagement, or other means of affecting capacity and the mass flow rates of the compressor. Thus, should the cooling demand on the refrigerant system be low, the compressor may be operated at a lower capacity to improve energy efficiency. While operating at a higher capacity, the refrigerant may well drive sufficient lubricant back from the condenser and evaporator to the compressor such that there is an adequate lubricant supply. However, at lower capacities, it may sometimes be difficult to adequately drive the lubricant back.

One known system periodically provides an increase in speed when the compressor is operating at the lower speed. The increased speed is maintained for a short period of time to drive lubricant back to the compressor. However, the increase in speed is not tied to any existing conditions in the refrigerant system, and thus may be operated too frequently, or not frequently enough. Of course, operating at the increased speed too frequency somewhat defeats the purpose of operating the compressor at the lower speed. On the other hand, running at the higher speed to return lubricant too infrequently would be even more undesirable.

SUMMARY OF THE INVENTION

In a disclosed embodiment of this invention, system conditions are monitored to predict an amount of oil migration from the compressor during low capacity operation. When a particular amount of lubricant is deemed to have migrated from the compressor, an increased compressor capacity is run for a period of time. As an example of a particular application of the invention, a variable speed compressor is used.

These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a refrigerant system.

FIG. 2 is a chart showing oil loss per hour at various compressor speeds.

FIG. 3 is a flowchart.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A refrigerant system 20 is illustrated in FIG. 1. A compressor shell 23 includes a lubricant sump 22, which maintains a quantity of lubricant. A compressor pump unit 24 compresses refrigerant and delivers the refrigerant to a discharge tube 26. From the discharge tube 26, the lubricant passes through a condenser 28, an expansion device 30, an evaporator 32, and back through a suction tube 33 into the compressor shell 23.

A control 36 for a compressor motor 37 may operate the compressor motor 37 at various speeds. Thus, the compressor may be operating at a relatively low speed to increase energy efficiency when a cooling demand is also low.

Sensors 34 and 35 monitor a condition, such as pressure, at the evaporator 32 and condenser 28, respectively. Alternatively, sensors that monitor temperature or an estimated saturated refrigerant temperature can be used to detect the conditions the compressor is operating at, These conditions may be sent to the control 36 to assist in quantifying the amount of oil which has likely migrated outwardly of the compressor shell 23 into other system components at low speed operation.

As shown in FIG. 2, a curve may be developed which equates oil loss for a period of time for various low compressor speeds. Thus, as shown, at some point, here 2400 rpm, the oil loss no longer occurs. However, at other speeds, there is pronounced oil loss.

The present invention integrates the amount of oil loss over time based upon a relationship such as shown in FIG. 2, and actuates the motor control 36 to increase the speed of the motor for a short period of oil return time when that integrated amount passes a particular limit.

Thus, as shown in the flowchart of FIG. 3, the amount of oil loss is calculated and the speed is increased when the amount passes a limit.

In one embodiment, the compressor speed may be ramped up from the low energy efficiency speed to a predetermined amount, e.g., 2400 rpm in a compressor having the characteristics as shown below, for a short period of time (e.g. 3 seconds).

The point at which the oil return speed-up will occur can be defined as a function of the pressure at the suction and discharge of the compressor, as a function of compressor capacity or based upon other variables. A ten coefficient map can be utilized to set a curve similar to that shown in FIG. 2.

Thus, the oil loss rate may be calculated as:

oil loss rate (oz/hr)=a1+a2(PE)+a3(PC)+a4(PE)² +a5(PE)(PC)+a6(PC)² +a7(PE)³ +a8(PE)²(PC)+a9(PE)(PC)² +a10(PC)³.

The oil loss rate shown above is based upon the pressure at the evaporator and the pressure at the condenser. The same form of equation could be applied to evap and condensing temperatures. It could also be multiplied by a multiplier which brings in a capacity level factor. One such multiplier factor may be determined by the following equation: Multiplier factor=b1(Rc)̂3+b2(Rc)̂2+b3(Rc)+b4.

Where Rc=Current capacity level/Max capacity level

As an example of this, for variable speed compressors this would be the current rpm/max RPM.

With the present invention, and once an adequate curve is developed, the quantity of oil which is “lost” or which has migrated from the compressor is calculated, or integrated, over time while the compressor is operating at a low capacity. Once that quantity exceeds a predetermined limit, then the compressor capacity is ramped up to the oil return speed for a short period of time.

In one application, the amount of oil lost is calculated in a plurality of discrete time units when the compressor is operating at a lower speed. As an example, this can occur every five seconds.

Although an embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention. 

1. A compressor comprising: a compressor pump unit, and said compressor pump unit and said motor being housed within a shell, said shell having a sump for maintaining a quantity of lubricant; a control for said compressor, said control being programmed to operate at at least one low capacity and at least one higher capacity, and said control also being operable to estimate an amount of oil which will have migrated from said compressor shell at least when said compressor is operating at said at least one lower capacity, and said motor control being operable to move said compressor motor from the at least one lower capacity to the at least one higher capacity once the estimated quantity of oil exceeds a predetermined limit.
 2. The compressor as set forth in claim 1, wherein said at least one lower capacity and said at least one higher capacity are achieved by varying the speed of the motor.
 3. The compressor as set forth in claim 1, wherein said estimate of the amount of oil is based upon pressures in the system.
 4. The compressor as set forth in claim 3, wherein the amount of oil estimate is based upon the pressures at both a condenser and an evaporator that are connected to said compressor.
 5. The compressor as set forth in claim 3, wherein the amount of oil estimate is based upon the estimated saturated temperature at both a condenser and an evaporator that are connected to said compressor
 6. The compressor as set forth in claim 1, wherein said estimate of the amount of oil is based upon temperature measurements.
 7. The compressor as set forth in claim 1, wherein a multiplier factor which includes a Capacity ratio quantity multiplied by an oil loss rate calculated from the pressures or estimate of saturated temperatures at said condenser and said evaporator.
 8. A refrigerant system comprising: a condenser, an evaporator, an expansion device and a compressor; the compressor having a motor for driving a compressor pump unit, and said compressor pump unit and motor being housed within a shell, said shell having a sump for maintaining a quantity of lubricant, a control for said motor, said control being programmed to operate at at least one low capacity and at least one higher capacity, and said control also being operable to estimate an amount of oil which will have migrated from said compressor shell at least when said compressor is operating at said at least one lower capacity, and said motor control being operable to move said compressor motor from the at least one lower capacity to the at least one higher capacity once the estimated quantity of oil exceeds a predetermined limit.
 9. The refrigerant system as set forth in claim 8, wherein said estimate of the amount of oil lost is based upon pressures in the system.
 10. The refrigerant system as set forth in claim 8, wherein the amount of oil estimate is based upon the pressures at both the condenser and the evaporator.
 11. The refrigerant system as set forth in claim 8, wherein a multiplier factor which includes an RPM quantity is multiplied by an oil loss rate calculated from the pressures at said condenser and said evaporator.
 12. The refrigerant system as set forth in claim 8, wherein said at least one lower capacity and said at least one higher capacity are achieved by varying the speed of the motor.
 13. A method of operating a compressor comprising the steps of: (a) operating a motor for a compressor at at least one low capacity and at least one higher capacity; (b) estimating an amount of oil which will have migrated from a compressor shell at least when said compressor is operating at said at least one lower capacity rate; and (c) moving said motor from the at least one lower capacity to the at least one higher capacity once the estimated quantity of oil exceeds a predetermined limit.
 14. The method as set forth in claim 13, wherein said estimate of the amount of oil lost is based upon pressures in the system.
 15. The method as set forth in claim 14, wherein the amount of oil estimate is based upon the pressures at both a condenser and an evaporator that are connected to said compressor.
 16. The method as set forth in claim 13, wherein a multiplier factor which includes an RPM quantity is multiplied by an oil loss rate calculated from the pressures at said condenser and said evaporator.
 17. The method as set forth in claim 13, wherein said at least one lower capacity and said at least one higher capacity are achieved by varying the speed of the motor
 18. The method as set forth in claim 13, wherein said estimate of the amount of oil is based upon temperature measurements. 